Anything hinting towards an awareness of beings as other than nothing-but-commodity might be called authentically 'religious.' At its best, black and death metal can be authentically religious.
Where black metal lapses into revivalism (enslaved, bathory, some burzum etc) it functions similarly to surrealism, artificially preserving 'nuggets' of once-everyday things, values and people in a distorted capsule. In the instance of metal, this distortion is found both literally in the guitar tone and destructively in its false anachronistic sensibilities. This is the phenomenon of 'museum-metal,' amusingly beloved of pagan-avatar-sporting metal-scene masses, and modern day Vikings alike.
Revivalist black metal is, I have argued recently, Judaic in essence in that it maps history to a messianic narrative-mythology*. Only in the coming of the messiah, the superman, a source of universal truths and understanding can history be understood from the perspective of the preserved nugget. Only by a messianic ordering, a messianic remembering can we, as Blake put it, see a world in a grain of sand. Cultural idealism appeals for validation towards the court of universal truth, whereby it can be elected as most valid; which I guess leaves revivalist art one half of a dialectic arch, forever limply thrashing for supportive validation.
* This raises the immense question of "authentic" religion as against(?) teleological narratives in which all beings are perhaps commodified as being-towards-a-culmination. It may even be that theistic religions are not authentically religious.